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Weirs
P R O T O C O L S | 385
W E I R S
Weirs
Christian E. Zimmerman and Laura M. Zabkar
Background and Objectives
Weirswhich function as porous barriers built across streamhave long been
used to capture migrating sh in owing waters. For example, the Netsilik peoples
of northern Canada used V-shaped weirs constructed of river rocks gathered on-
site to capture migrating Arctic char Salvelinus alpinus (Balikci 1970). Similarly,
fences constructed of stakes and a latticework of willow branches or staves were
used by Native Americans to capture migrating salmon in streams along the West
Coast of North America (Stewart 1994). In modern times, weirs have also been
used in terminal sheries and to capture brood sh for use in sh culture. Weirs
have been used to gather data on age structure, condition, sex ratio, spawning
escapement, abundance, and migratory patterns of sh in streams.

One of the critical elements of sheries management and stock assessment of
salmonids is a count of adult sh returning to spawn. Weirs are frequently used to
capture or count sh to determine status and trends of populations or direct in-
season management of sheries; generally, weirs are the standard against which
other techniques are measured.

To evaluate shery management actions, the number of sh escaping
to spawn is often compared to river-specic target spawning requirements
(OConnell and Dempson 1995). A critical factor in these analyses is the
determination of total run size (OConnell 2003). OConnell compared methods of
run-size estimation against absolute counts from a rigid weir and concluded that,
given the uncertainty of estimators, the absolute counts obtained at the weir wer
signicantly better than modeled estimates, which deviated as much as 5060%
from actual counts. The use of weirs is generally restricted to streams and small
rivers because of construction expense, formation of navigation barriers, and
the tendency of weirs to clog with debris, which can cause ooding and collapse
of the structure (Hubert 1996). When feasible, however, weirs are generally
regarded as the most accurate technique available to quantify escapement as the
result is supposedly an absolute count (Cousens et al. 1982). Weirs also provide
the opportunity to capture sh for observation and sampling of biological
characteristics and tissues; they may also serve as recapture sites for basin-wide,
markrecapture population estimates. Temporary weirs are useful in monitoring
wild populations of salmonids as well as for capturing broodstock for articial
propagation. 386 | P R O T O C O L S
W E I R S
FIGURE 1. A temporary weir constructed of metal tripods, stringers,
and galvanized conduit with both an upstream and downstream trap.
Ninilchik River, Alaska.
Rationale
Temporary weirs enable eld biologists to quantify the escapement of adult
salmonids in streams and rivers. In addition to providing absolute counts of sh
migrating through the weir, weirs can be used to capture sh, determine sex
ratios and species composition, recapture tagged sh, and collect tissue or scale
samples. In locations where management relies on escapement goals, weirs can
be used to monitor escapement in order to direct in-season management of
commercial or subsistence sheries.
Objectives
This protocol describes the methods used in counting migrating adult
salmonids in streams and rivers using weirs and traps. Generally, weirs and traps
are temporarily installed across the stream channel and enable monitoring
practitioners to estimate or make an absolute count of sh passing that point in
the stream.
FIGURE 2. A resistance board weir featuring a ski gate to allow boat passage (in the foreground,
marked by pylons), a trap to capture sh in middle of weir, and a passage chute (background), where sh
are counted as they pass the weir. Pilgrim River, Alaska. (Photo: Karen Dunmall and Tim Kroeker.) P R O T O C O L S | 387
W E I R S

Temporary weirs may be constructed from a range of materials. Rigid weirs
generally consist of a fence and support structure; fences may be constructed from
netting (Blair 1956; Noltie 1987) or from rigid material such as pipe or metal pickets
(Hill and Matter 1991). These weirs are generally easy to dismantle and transport
but are sometimes dicult to maintain during high water or in streams with high
debris loads. Weirs constructed of screen or wire panels have a tendency to collect
debris such as leaves and algae (Clay 1961). Kristoerson et al. (1986) used a weir
constructed of polyethylene (Vexar®) and metal t-posts, similar to that described
by Noltie (1987), on an Arctic river but found that clogging by algae and debris
led to excessive water pressure that eventually caused the weir to collapse. Noltie
(1987), however, reported that the same material could easily be cleaned of leaves
using a push broom. Because weirs constructed of these materials are relatively
inexpensive, they are probably best used for short-term studies in small shallow
streams; practitioners choosing these materials need to take into account the time
needed to clean these weirs of debris.

Weirs constructed of metal pickets (which are frequently made of aluminum
rods or galvanized conduit) are more resistant to buildup of algae, leaves, and
other ne material. In some designs, the pickets can be removed for easy cleaning
and to reduce pressure from high ows. The length of the conduit will depend
on the depth of the stream and should be long enough that salmon should not
be expected to jump over the weir. Anderson and McDonald (1978), Kristoerson
et al. (1986), and Hill and Matter (1991) describe construction details for such
rigid weirs, which generally consist of structures that support panels of pickets.
Supports usually consist of tripods constructed of pipe or wood and support
stingers that hold the pickets (see Figure 3). By angling the upstream face of the
weir at 120° relative to the stream bottom, the water ows slightly up the pickets
before passing through; this movement creates a greater area over which water
pressure may dissipate (Anderson and McDonald 1978). To further dissipate water
pressure, the weir can be constructed so that the wings of the weir terminate in a
90° angle entering the trap box (see Figure 4). This arrangement allows more water
to pass through the weir for a given stream width and guides sh into the trap.
Mullins et al. (1991) describe a two-way trap that can be constructed in the apex of
two weirs that allows for sampling of both upstream and downstream migrants.
FIGURE 3. Construction details of tripods and weir panels made of metal stringers and pickets of
galvanized electrical conduit. 388 | P R O T O C O L S
W E I R S

Rigid weirs work best in rivers that have minimal variation in water ow and
depth; these conditions will help avoid, to the greatest extent possible, frequency
of washout of the trap and/or weir by increased ows or seasonal freshets. Lake
outlets, therefore, are particularly suited to the placement of rigid weirs (Clay
1961). Rigid weirs are also susceptible to damage by large oating debris such as
logs or ice. Resistance board weirs were designed to accommodate uctuation
in ow and debris and to allow for inclusion of easy-to-use boat passes (Tobin
1994; Stewart 2002) (see gures 2 and 4). Although not impervious to washout,
resistance board weirs can be used in rivers that experience debris-laden high
water periods (Tobin 1994). During high water, the resistance board weir will
temporarily submerge when pressure created by water and debris loading
reaches a point that would typically wash a rigid weir downstream (Tobin 1994).
This exibility requires less maintenance and also reduces the frequency of these
occasions when sh cannot be counted.

Resistance board weirs consist of three main components: panels made of
capped polyvinyl chloride (PVC) pipe, a rail anchored to the substrate that attaches
the panels to the river bottom, and a trap box or chute where sh are captured or
counted. Detailed construction and installation manuals for resistance board weirs
are available in Tobin (1994) and Stewart (2002, 2003). In summary, a rail is installed
across the stream. This rail is anchored to the substrate using steel rod and cables
attached to duck-bill anchors placed upstream of the weir. The rail anchors and
aligns the cable to which panels are attached. The weir panels are constructed of
schedule 40 PVC electrical conduit 6.1 m in length. Electrical conduit is used rather
than PVC water pipe because it resists breakdown caused by ultraviolet light.
Panels consist of multiple pipes supported by 1.2 m-long stringers that are spaced
evenly lengthwise along the panels to prov